The present invention relates generally to a strain sensor and more particularly to an optical fiber strain-to-failure sensor which can measure small amounts of strain.
Oftentimes it is desirable to determine whether a structure has exceeded some maximum strain value during a given time frame, as a means of providing a warning of failure of the structure. For example, it may be desirable to know if some component in an aircraft has exceeded some predetermined strain value during a flight.
Optical fiber strain sensors exist which rely on failure of the optical fiber at the location of strain to indicate the presence of the strain. In such a sensor, the fiber is attached to the structure so that when the structure experiences strain the fiber experiences the same amount of strain. When the structure has reached an amount of strain that is the same as the fiber's maximum strain capacity, the fiber fails. The failure of the fiber is detected when light is launched into one end of the fiber and not detected at the other end. Such sensors have the advantage that they are lightweight and do not take up much space. Furthermore, they can be embedded in structures like laminated composites. One problem with sensors of this type is that the amount of strain which will cause the fiber in such a sensor to fail is variable, making it difficult to predetermine the maximum strain to an accurate degree. Additionally, the amount of strain required to break the fiber is too large to be of interest in many applications. Most optical fibers will not fail until they have reached a strain level in the range of 20,000 to 30,000 microstrain, yet structures may fail at strain levels as low as 2000 to 3000 microstrain.